The invention relates to a bistable electromagnet, particularly an electromagnetic valve which is actuated by an electromagnet having a yoke. A coil is connected to the yoke. The coil has an interior region. An element is located within the interior region of the coil and defines a hollow space therein. An armature is provided that is movable between two operating positions. A spring acts upon the armature. A permanent magnet is arranged in the hollow space of the element and holds the armature against a force of the spring in one of the operating positions. An electromagnet of this type usually has a cylindrical coil and is frequently called a plunger-type magnet. The yoke can contain a hollow cylinder surrounding the coil or consist of a flat material bent in a U-shape.
A known bistable electromagnet is described in European Patent Application 0 219 572. It actuates a vacuum valve and is distinguished by the fact that the entire coil core is constructed as a permanent magnet and is stationarily associated with the armature. The armature plunges into the coil by means of a positive direct-current pulse and is held in this attracted operated position by the permanent magnet in opposition to the spring force. A negative direct-current pulse weakens the holding force of the permanent magnet to such a degree that the spring force predominates and the armature goes into the released operating position.
However, the holding force of the permanent magnet is very sensitive to tolerance in the attracted operating position, that is to say the unavoidable production tolerances of the permanent magnet and the other components make an individual mechanical adjustment or adjustment of the period or amplitude of the control pulse unavoidable in series production. A further disadvantage of the known magnet consists in that the release pulse must be long enough for the armature to be no longer within the range of influence of the permanent magnet. The control power is therefore undesirably high in spite of the basically economic operation.